The neuronal cytoskeleton is intimately involved in maintaining the size and shape of axons and dendrites. The proposed studies examine the transport and organization of neurofilaments within the axonal cytoskeleton. The experiments aim to given new insight into normal developmental processes and into pathological processes which may be pertinent to human disorders such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophies, peripheral neuropathies, and Alzheimer's disease. The first studies concern the mechanisms of radial growth of axons in sciatic nerve during development. Previous studies have shown that neurofilaments occupy most of the volume of large axons, and that neurofilament number and axonal cross-sectional area are closely related throughout development, suggesting that neurofilament delivery is likely to regulate axonal growth. The proposed studies assess the relative contribution of increased neurofilament synthesis and retardation of neurofilament transport along the axon for determining radial growth. The studies also will assess the changes in neurofilament synthesis and transport and correlate them with axonal size during the atrophy caused by axotomy. The post-translational modification of neurofilaments, for example, phosphorylation, will be investigated and correlated with changes in the velocity of neurofilament transport. A different set of studies will examine transport of cytoskeletal proteins in selected CNS pathways, using techniques developed in the PNS. These studies will provide the necessary foundations for examining neurofilament transport in the disorders marked by neurofibrillary degeneration. A third set of studies is stimulated by the observation that material can be obtained from peripheral nerve that is highly enriched in neurofilaments which are organized into large bundles. These studies will investigate molecular interactions are responsible for the interfilament links in such preparations, with the aim of giving insight into the factors controlling interfilament linking in normal and pathologic states.